Selective termination of wireless connections to refresh signal information in wireless node location infrastructure

ABSTRACT

Methods, apparatuses and systems directed to refreshing signal information in an infrastructure wireless node location mechanism. The wireless node location mechanism selectively terminates connections with wireless clients to refresh signal strength information used to compute an estimated location for the wireless clients. The wireless node location mechanism terminates the connection between a WLAN and a given wireless node, causing in typical WLAN protocol implementations, the mobile station to transmit frames or packets on all available operating channels in a given band. This allows access points and other WLAN elements, operating on different frequency channels, to detect frames transmitted by the mobile station and provide refreshed signal strength information to a wireless node location mechanism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application makes reference to the following commonly owned U.S.patent applications and/or patents, which are incorporated herein byreference in their entirety for all purposes:

-   -   U.S. patent application Ser. No. 10/155,938 in the name of        Patrice R. Calhoun, Robert B. O'Hara, Jr. and Robert J. Friday,        entitled “Method and System for Hierarchical Processing of        Protocol Information in a Wireless LAN;”    -   U.S. application Ser. No. 10/183,704 in the name of Robert J.        Friday, Patrice R. Calhoun, Robert B. O'Hara, Jr., Alexander H.        Hills and Paul F. Dietrich, and entitled “Method and System for        Dynamically Assigning Channels Across Multiple Radios in a        Wireless LAN;”    -   U.S. patent application Ser. No. 10/407,357 in the name of        Patrice R. Calhoun, Robert B. O'Hara, Jr. and Robert J. Friday,        entitled “Method and System for Hierarchical Processing of        Protocol Information in a Wireless LAN;”    -   U.S. patent application Ser. No. 10/407,370 in the name of        Patrice R. Calhoun, Robert B. O'Hara, Jr. and David A. Frascone,        entitled “Wireless Network System Including Integrated Rogue        Access Point Detection;”    -   U.S. application Ser. No. 10/447,735 in the name of Robert B.        O'Hara, Jr., Robert J. Friday, Patrice R. Calhoun, and Paul F.        Dietrich and entitled “Wireless Network Infrastructure including        Wireless Discovery and Communication Mechanism;” and    -   U.S. application Ser. No. 10/611,660 in the name of Paul F.        Dietrich, David A. Frascone, Patrice R. Calhoun, Robert J.        Friday, Robert B. O'Hara, Jr., and Matthew D. Howard and        entitled “Containment of Rogue Systems in Wireless Network        Environments.”

FIELD OF THE INVENTION

The present invention relates to locating wireless nodes in wirelessnetwork environments and, more particularly, to a wireless node locationmechanism that selectively terminates connections with wireless clientsto refresh signal strength information used to compute estimatedlocations for the wireless clients.

BACKGROUND OF THE INVENTION

Market adoption of wireless LAN (WLAN) technology has exploded, as usersfrom a wide range of backgrounds and vertical industries have broughtthis technology into their homes, offices, and increasingly into thepublic air space. This inflection point has highlighted not only thelimitations of earlier-generation systems, but the changing role WLANtechnology now plays in people's work and lifestyles, across the globe.Indeed, WLANs are rapidly changing from convenience networks tobusiness-critical networks. Increasingly users are depending on WLANs toimprove the timeliness and productivity of their communications andapplications, and in doing so, require greater visibility, security,management, and performance from their network.

The rapid proliferation of lightweight, portable computing devices andhigh-speed WLANs has enabled users to remain connected to variousnetwork resources, while roaming throughout a building or other physicallocation. The mobility afforded by WLANs has generated much interest inapplications and services that are a function of a mobile user'sphysical location. Examples of such applications include: printing adocument on the nearest printer, locating a mobile user, displaying amap of the immediate surroundings, and guiding a user inside a building.The required or desired granularity of location information varies fromone application to another. Indeed, the accuracy required by anapplication that selects the nearest network printer, or locates a rogueaccess point, often requires the ability to determine in what room amobile station is located. Accordingly, much effort has been dedicatedto improving the accuracy of wireless node location mechanisms.

The use of radio signals to estimate the location of a wireless deviceor node is known. For example, a Global Positioning System (GPS)receiver obtains location information by triangulating its positionrelative to four satellites that transmit radio signals. The GPSreceiver estimates the distance between each satellite based on the timeit takes for the radio signals to travel from the satellite to thereceiver. Signal propagation time is assessed by determining the timeshift required to synchronize the pseudo-random signal transmitted bythe satellite and the signal received at the GPS receiver. Althoughtriangulation only requires distance measurements from three points, anadditional distance measurement from a fourth satellite is used forerror correction.

The distance between a wireless transmitter and a receiver can also beestimated based on the strength of the received signal, or moreaccurately the observed attenuation of the radio signal. Signalattenuation refers to the weakening of a signal over its path of traveldue to various factors like terrain, obstructions and environmentalconditions. Generally speaking, the magnitude or power of a radio signalweakens as it travels from its source. The attenuation undergone by anelectromagnetic wave in transit between a transmitter and a receiver isreferred to as path loss. Path loss may be due to many effects such asfree-space loss, refraction, reflection, aperture-medium coupling loss,and absorption.

In business enterprise environments, most location-tracking systems arebased on RF triangulation or RF fingerprinting techniques. RFtriangulation calculates a mobile user's location based upon thedetected signal strength of nearby access points (APs). It naturallyassumes that signal strength is a function of proximity in computing thedistances between the wireless node and the access points. RFfingerprinting, on the other hand, compares a mobile station's view ofthe network infrastructure (i.e., the strength of signals transmitted byinfrastructure access points) with a database that contains an RFphysical model of the coverage area. This database is typicallypopulated by either an extensive site survey or an RF prediction modelof the coverage area. For example, Bahl et al., “A Software System forLocating Mobile Users: Design, Evaluation, and Lessons,”http://research.microsoft.com/˜bahl/Papers/Pdf/radar.pdf, describes anRF location system (the RADAR system) in a WLAN environment, that allowsa mobile station to track its own location relative to access points ina WLAN environment.

The RADAR system relies on a so-called Radio Map, which is a database oflocations in a building and the signal strength of the beacons emanatingfrom the access points as observed, or estimated, at those locations.For example, an entry in the Radio Map may look like (x, y, z, ss^(i)(i=1 . . . n)), where (x, y, z) are the physical coordinates of thelocation where the signal is recorded, and ss_(i) is the signal strengthof the beacon signal emanating from the ith access point. According toBahl et al., Radio Maps may be empirically created based on heuristicevaluations of the signals transmitted by the infrastructure radios atvarious locations, or mathematically created using a mathematical modelof indoor RF signal propagation. To locate the position of the mobileuser in real-time, the mobile station measures the signal strength ofeach access point within range. It then searches a Radio Map databaseagainst the detected signal strengths to find the location with the bestmatch. Bahl et al. also describe averaging the detected signal strengthsamples, and using a tracking history-based algorithm, to improve theaccuracy of the location estimate. Bahl et al. also address fluctuationsin RF signal propagation by using multiple Radio Maps and choosing theRadio Map which best reflects the current RF environment. Specifically,an access point detects beacon packets from other access points andconsults a Radio Map to estimate its location, and evaluates theestimated location with the known location. The RADAR system chooses theRadio Map which best characterizes the current RF environment, based ona sliding window average of received signal strengths.

While the RADAR system allows a mobile station to track its location, itdoes not disclose a system that allows the WLAN infrastructure to trackthe location of wireless nodes, such as rogue access points. Indeed, theuse of a WLAN infrastructure to collect signal strength informationcorresponding to a mobile station for use in estimating the location ofthe mobile station does present certain difficulties. The extremelyportable nature of mobile stations renders it important to possesssufficiently recent signal strength information for a given mobilestation, as it may have moved to a new location after one or more signalstrength measurements have been collected by the locationinfrastructure. In the RADAR system, this is not an issue since themobile station computes its own location based on beacon packets thataccess points regularly transmit as part of the normal access point modedefined by the 802.11 protocol. Accordingly, the mobile station can scanall available channels to obtain one or more beacon packets on thechannels, and then compute its location based on the newly detectedsignal strength. In the reverse situation where the WLAN collects signalstrength data from wireless nodes, collecting signal strength data canbe problematic, since mobile stations ordinarily do not regularlytransmit management frames, such as beacon packets, once they associatewith an access point. Moreover, adjacent access points in typical WLANenvironments operate on non-overlapping channels to exploit theadvantages associated with frequency re-use. Accordingly, access pointsadjacent to the access point to which a given mobile station isassociated will not be able to detect RF signals transmitted by themobile station, unless the adjacent access points go “off channel” todetect the signals transmitted by the mobile station. Switching to analternate channel to passively or actively scan for a given mobilestation interrupts connections with mobile stations associated with anaccess point. The lack of signal strength information from adjacentaccess points is especially problematic to wireless node location assignal strength measurements from adjacent access points are typicallythe most useful in locating a given mobile station. For example, thesignal strength information from adjacent access points is typicallymore accurate as the adjacent access points are generally closer inproximity to the mobile station. Still further, the lack of signalstrength information from a sufficient number of access points mayprevent the mobile station from being located entirely as locationmechanisms require signal strength information from a minimum number ofsources.

In light of the foregoing, a need exists in the art for methods,apparatuses and systems directed to refreshing signal strengthinformation in an infrastructure wireless node location mechanism. Inaddition, a need in the art exists for wireless node location mechanismsthat efficiently integrate into WLAN infrastructures. Embodiments of thepresent invention substantially fulfill these needs.

SUMMARY OF THE INVENTION

The present invention provides methods, apparatuses and systems directedto refreshing signal information in an infrastructure wireless nodelocation mechanism. According to an implementation of the presentinvention, the wireless node location mechanism selectively terminatesconnections with wireless clients to refresh signal strength informationused to compute an estimated location for the wireless clients. Thepresent invention takes advantage of the characteristics of mobilestations to refresh signal strength information to enhance the accuracyof wireless node location in a WLAN environment. As discussed below, thewireless node location mechanism terminates the connection between aWLAN and a given wireless node, causing in typical WLAN protocolimplementations, the mobile station to transmit frames or packets on allavailable operating channels in a given band. This allows access pointsand other WLAN elements, operating on different frequency channels, todetect frames transmitted by the mobile station and provide refreshed RFsignal information to a wireless node location mechanism.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram including a wireless node locationmechanism according to an implementation of the present invention.

FIG. 2A is a flow chart diagram illustrating the overall process flowdirected to the location of a wireless node according to animplementation of the present invention.

FIG. 2B is a flow chart diagram illustrating an overall process flow,according to an alternative implementation of the present invention,directed to locating a wireless node.

FIG. 3 is a functional block diagram illustrating a wireless networksystem according to an implementation of the present invention.

FIG. 4 is a functional block diagram highlighting the wireless nodelocation functionality of a central control element in the wirelessnetwork system of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

A. Wireless Node Location and Forced Termination of Wireless Connections

FIG. 1 illustrates the basic operating components of the wireless nodelocation mechanism according to an implementation of the presentinvention. As FIG. 1 shows, the wireless node location mechanismincludes a wireless node location module 59 and a plurality ofinfrastructure radio transceivers 58 disposed throughout a physicalspace. One skilled in the art will recognize that the system depicted inFIG. 1 represents an example of the basic components of the inventionand is mostly for didactic purposes. As discussed more fully below, thefunctionality generally denoted by infrastructure radio transceivers 58and wireless node location module 59 can be integrated into a variety ofsystems, such as wireless systems dedicated for location of wirelessnodes, or WLAN or other wireless network systems. For didactic purposes,the embodiments described below operate in connection with a WLANenvironment according to the IEEE 802.11 WLAN protocol. One skilled inthe art will recognize, however, that the present invention can beapplied to any suitable wireless network protocol, where mobile stationsoperate substantially as described herein.

Infrastructure radio transceivers 58 generally comprise at least oneantenna, a radio transmit/receive unit, and control logic (e.g., a802.11 control unit) to control the transmission and reception of radiosignals according to a wireless communications protocol. Infrastructureradio transceivers 58, in one implementation, are disposed in knownand/or fixed locations throughout a physical space, such as a room, acollection of rooms, a floor of a building, an entire building, or anarbitrarily-defined region, including outside environments, over whichinfrastructure radio transceivers 58 provide radio-frequency (RF)coverage.

A.1. Infrastructure Radio Transceiver

Infrastructure radio transceivers 58 are operative to detect thestrength of received radio-frequency signals, such as the signals 57transmitted by wireless node 56 and by other radio transceivers, andprovide the detected signal strength data for corresponding wirelessnodes to wireless node location module 59. In one implementation,infrastructure radio transceivers 58 are also operative to transmit andreceive wireless or radio-frequency signals according to a wirelesscommunications protocol, such as the IEEE 802.11 WLAN protocol.Infrastructure radio transceivers 58, in one implementation, can operateon a selected channel from a plurality of channels in a given band. Inanother implementation, infrastructure radio transceivers 58 can alsooperate in more than one band. For example, infrastructure radioreceivers 58 may be configured to operate in the 802.11a-5 GHz band, the802.11b/g-2.4 GHz band, or both. In one implementation, infrastructureradio transceivers 58 can be configured to collect the signal strengthinformation associated with wireless nodes and transmit the collecteddata in response to SNMP or other requests by wireless node locationmodule 59. As discussed below, other methods for collecting signalstrength data may also be employed.

Identification of wireless nodes depends on the wireless communicationsprotocol in use. For 802.11 WLAN environments, for example, wirelessnodes can be identified based on MAC address. Furthermore, wirelessnodes can be authorized mobile stations, such as remote client elements16, 18 (see FIG. 3), rogue systems (e.g., rogue access points and/orrogue mobile stations), as well as authorized access points for which nolocation information is known. In other implementations, wireless nodescan be identified based on a unique property of the RF signal, such as agiven frequency channel, or a unique signal pattern, and the like. Forexample, the wireless node location functionality may be employed tolocate a detected source of interference, such as a non-802.11 compliantdevice.

In one implementation, infrastructure radio transceivers 58 are alsooperable to communicate with one or more mobile stations, such aswireless node 56, according to a wireless communication protocol. Forexample, each infrastructure radio transceiver 58, in oneimplementation, is an access point or other WLAN component. In oneimplementation, radio transceiver 58 is operably connected to a LocalArea Network (LAN), Wide Area Network (WAN) or other wireline network tobridge traffic between mobile stations and the wireline network. Asdiscussed more fully below, infrastructure radio transceiver 58 may alsobe an access element or light weight access point in a wireless networkfeaturing hierarchical processing of protocol information. U.S. patentapplication Ser. No. 10/155,938, incorporated by reference above,discloses light weight access points in connection with hierarchicalprocessing of wireless protocol information. In one implementation, theradio transceiver 58 implements the 802.11 protocols (where 802.11, asused herein, generically refers to the IEEE 802.11 standard for wirelessLANs and all its amendments). Of course, the present invention can beused in connection with any suitable radio-frequency-based wirelessnetwork or communications protocol.

For purposes of describing an embodiment of the present invention,infrastructure radio transceivers 58, in one implementation, arewireless access points associated with a wireless LAN based on the IEEE802.11 standard. When a mobile station, such as wireless node 56,initializes or moves into a new coverage area, according to the 802.11standard, it transmits probe requests on all operating channels in agiven band to locate access points to which it may associate toestablish a wireless connection. Mobile stations that are capable oftransmitting and receiving radio signals in different bands (e.g., dualband mobile stations) can transmit probe requests on all availablechannels in all bands. The mobile station scans the available channelsin all available bands in the region and listens to Beacon Frames orProbe Response Frames transmitted by access points in that region.Especially in enterprise WLAN systems, the mobile station may oftendetect multiple access points transmitting in one or more bands. Afterthe mobile station selects a given access point and a band, it sends anauthentication frame containing a wireless node identifier (in 802.11environments, a MAC address associated with the radio Network InterfaceController (NIC) of the mobile station) to the access point. With opensystem authentication, the mobile station transmits only oneauthentication frame, and the access point responds with anauthentication frame as a response indicating acceptance (or rejection).With shared key authentication, the radio NIC of the mobile stationsends an initial authentication frame, and the access point respondswith an authentication frame containing challenge text. The mobilestation must send an encrypted version of the challenge text (using itsWired Equivalent Privacy (WEP) key) in an authentication frame back tothe access point. The access point ensures that the mobile station hasthe correct WEP key by seeing whether the challenge text recovered afterdecryption is the same that was sent previously. Based on the results ofthis comparison, the access point replies to the mobile station with anauthentication frame signifying the result of authentication. Otherauthentication schemes may also be employed, such as 802.1x.

After authentication, the mobile station transmits an associationrequest frame to the access point. 802.11 association enables the accesspoint to allocate resources for and synchronize with the radio networkinterface controller (NIC) of the mobile station. The associationrequest frame carries information about the radio NIC (e.g., supporteddata rates) and the Service Set Identifier (SSID) of the network withwhich it wishes to associate. After receiving the association request,the access point considers associating with the radio NIC, and (ifaccepted) reserves memory space and establishes an association ID forthe radio NIC. An access point sends an association response framecontaining an acceptance or rejection notice to the radio NIC requestingassociation. If the access point accepts the radio NIC, the frameincludes information regarding the association, such as association IDand supported data rates. If the outcome of the association is positive,the radio NIC can utilize the access point to communicate with othermobile stations on the network and systems on the distribution (e.g.,Ethernet) side of the access point. In one implementation, the accesspoint stores information about the mobile station in an associationtable, including the MAC address of the mobile station and theassociation ID. In one implementation, remote devices, such as wirelessnode location module 59, may access information in the association tablevia an suitable query method. For example, the draft standard IEEE802.11k defines a Management Information Base (MIB), that holds theassociation table which is accessible via SNMP queries. Of course, otherproprietary access methods may also be employed.

In one implementation, infrastructure radio transceivers 58 make use ofthe signal strength detection functionality residing on a wirelessnetwork interface adapter. For example, the IEEE 802.11 standard definesa mechanism by which RF energy is measured by the circuitry (e.g., chipset) on a wireless network adapter or interface card. The IEEE 802.11protocol specifies an optional parameter, the receive signal strengthindicator (RSSI). This parameter is a measure by the PHY layer of theenergy observed at the antenna used to receive the current packet orframe. RSSI is measured between the beginning of the start framedelimiter (SFD) and the end of the Physical Layer Convergence Procedure(PLCP) header error check (HEC). This numeric value is an integer withan allowable range of 0-255 (a 1-byte value). Typically, 802.11 chip setvendors have chosen not to actually measure 256 different signal levels.Accordingly, each vendor's 802.11-compliant adapter has a specificmaximum RSSI value (“RSSI_Max”). Therefore, the RF energy level reportedby a particular vendor's wireless network adapter will range between 0and RSSI_Max. Resolving a given RSSI value reported by a given vendor'schip set to an actual power value (in dBm) can be accomplished byreference to a conversion table. In addition, some wireless networkingchip sets actually report received signal strength in dBm units, ratherthan, or in addition to, RSSI. Other attributes of the signal can alsobe used in combination with received signal strength or as analternative. For example, the detected Signal-to-Noise Ratio (SNR)during packet reception can be used in determining overlay signaltransmit power. Again, many chip sets include functionality andcorresponding APIs to allow for a determination of SNRs associated withpackets received from other transceivers 58 and/or wireless node 56. Inone implementation, infrastructure radio transceivers 58 stores signalstrength data corresponding to the last received frame in an extendedassociation table. In one such implementation, the association table isfurther extended to include a time stamp indicating the time of the lastreceived frame. Accordingly, the signal strength values and time stampswill be overwritten as new frames are received. In otherimplementations, this information can be stored in a separate table orother data structure. In another implementation, the association tableto support dual-band configuration can further be extended to include anidentifier for the band (e.g., 2.4 v. 5 GHz band) on which the frame wasreceived. In addition, as described below, the signal strengthinformation may be collected at another device.

A.2. Wireless Node Location Module

Wireless node location module 59, in one implementation, collects signalstrength data received from infrastructure radio transceivers 58 andmaintains the signal strength data in association with a wireless nodeidentifier, and an identifier for the infrastructure radio transceiver58 which provided the signal strength data. In one implementation, thesignal strength data may also include the frequency band associated withthe channel on which the frame was detected. Wireless node locationmodule 59, in one implementation, is also configured to distinguishbetween signals received from infrastructure radio transceivers 58 andsignals received from other wireless nodes based on the wireless nodeidentifier. In one implementation, wireless node location module 59maintains a variety of data structures for storing signal strengthinformation. For example, one data structure is used to store the signalstrength of signals transmitted between infrastructure radiotransceivers 58. In one implementation, wireless node location module 59stores this inter-IRT signal strength data in a N×N IRT matrix, where Nis the number of infrastructure radio transceivers 58. The columnentries can correspond to the transmitting transceiver, while the rowentries correspond to the receiving transceiver, or vice versa. Variousentries in this matrix may be null values as all infrastructure radiotransceivers may not, and in most deployments probably will not, be ableto detect one another. This inter-IRT signal strength data can be usedfor a variety of purposes, such as updating one or more parametersassociated with the location algorithm, or calibrating signal strengthdetection across the infrastructure radio transceivers 58.

Wireless node location module 59, in one implementation, maintainssignal strength data for all other wireless nodes in tables or othersuitable data structures. In one implementation, wireless node locationmodule 59 maintains, for each radio transceiver 58, a separate tableincluding at least two fields: 1) a wireless node identifier; and 2) thedetected signal strength. Additional fields may also include: 1) a timestamp indicating the time the radio transceiver 58 received the signal,2) a channel identifier, and/or 3) a frequency band identifier. In oneimplementation, when the memory space allocated to the wireless nodetables is depleted, the least recently used/updated entry as indicatedby the time stamps is overwritten. In one implementation, wireless nodelocation module 59 filters the signal strength data received from theinfrastructure radio transceivers 58 against a list of wireless nodeidentifiers in order to identify the appropriate data structure toupdate. One skilled in the art will recognize that a variety of datastructures beyond matrices and tables can be used.

As discussed above, signal strengths are detected, in oneimplementation, on a frame-by-frame basis. Accordingly, in oneembodiment, the signal strength data maintained by wireless nodelocation module 59 can be updated as the frames/packets are received. Inone implementation, the latest signal strength value is used toessentially overwrite the old value. In other implementations, however,an average, moving average or weighted moving average can be used ifsuccessive wireless frames corresponding to a given wireless node areencountered within a threshold time interval (e.g., typically resultingfrom a data stream transmission). In such a situation, the time stampcan correspond to the time of the last packet or frame. In addition,while radio transceivers 58 when operating as access points typicallyoperate on different channels, mobile stations at various times (e.g.,transmitting probe requests to find access points) transmit wirelessframes on all available operating channels. This helps to ensure that aplurality of radio transceivers 58 detect the mobile station. In someimplementations, one or more infrastructure radio transceivers 58 thatare adjacent to a radio transceiver 58 that detected a given wirelessnode may be directed to switch to a given operating channel to listenfor signals transmitted by the mobile station. Still further, asdiscussed below, the infrastructure radio transceivers 58 may becommanded to specifically transmit frames on a given channel for thepurpose of updating the signal strength data maintained by wireless nodelocation module 59.

Wireless node location module 59, in one implementation, also maintainsa RF physical model of the coverage area associated with the RFenvironment, and uses an RF fingerprinting algorithm to compute theestimated location of a wireless node. As discussed in more detailbelow, the RF physical model returns an estimated physical location of awireless node, given the strength of signals detected by theinfrastructure radio transceivers 58, as well as an indication of theinfrastructure radio transceivers reporting the signal strengths. The RFphysical model can be based on any suitable location model that usessignal strength to determine the location of a wireless node. Forexample, the RF physical model may be based on site survey data, RFprediction computations, or a combination of the two.

In one implementation, the RF physical model characterizes for eachinfrastructure radio transceiver 58 the received signal strengthassociated with a wireless transmitter at different locations. Forexample, in one implementation, the RF physical model comprises a radiocoverage map or matrix that indicates the expected signal strengthreceived from a wireless node, given a uniform transmit power, at agiven location defined in x-, and y-coordinates. This database can bepopulated in a variety of ways. For example, the radio coverage maps canbe populated with the results of an extensive site survey, according towhich a wireless transmitter is placed at different locations in thephysical space. During the site survey, the infrastructure radiotransceivers 58 operate in a listening mode and report the resultingsignal strength of the signal transmitted by the wireless node used toconduct the site survey. In one implementation, the infrastructure radiotransceivers 58 can be configured to transmit the signal strength databack to the wireless transmitter, which may be a laptop computer orother wireless device. The coverage maps are constructed by associatingthe signal strength and location data in the coverage maps correspondingto each infrastructure radio transceiver. The coverage maps may also beconstructed by having the WLAN tester (or other wireless node) simplymeasure the signal strength of frames transmitted by the infrastructureradio transceivers 58 (e.g., beacon packets) at desired locations withinthe physical location. If path loss symmetry is assumed, this values canbe used to construct the coverage maps for each of the infrastructureradio transceivers. To estimate the location of the wireless node,wireless node location module 59 determines the location coordinates, orrange of location coordinates, that best fit the coverage mapsassociated with the infrastructure radio transceivers 58 selected tolocate the wireless node based on the detected signal strength data, asdiscussed in more detail below.

In one implementation, a coverage map, for each infrastructure radiotransceiver 58, is maintained that includes the signal strengths in anN×M matrix, where N is the number of x-coordinates in the coverage map,and M is the number of y-coordinates in the coverage map. In oneimplementation, the extent of the physical space model by the coveragemaps for each infrastructure radio transceiver 58 are co-extensive. Thecoverage maps for all infrastructure radio transceivers 58 can beco-extensive with the physical space in which the location system isdeployed, or with a boundary configured by a network administrator. Inone implementation, however, knowledge of various antenna attributesassociated with each infrastructure radio transceiver 58—such as antennatype (e.g., omni-directional, directional), peak gain orientation,beamwidth, front-to-back isolation—can be used to compress or reduce thesize of the coverage maps. In one implementation, the coverage maps canbe configured to be substantially co-extensive with the antenna patternof each antenna connected to the infrastructure radio transceivers 58out to a threshold signal strength or gain level. For example, thecoverage map for a given antenna can be compressed to the front orintended coverage area of the directional antenna. Of course, other datastructures can be used such as a table including location coordinatesstored in association with tuples of signal strengths and infrastructureradio transceiver antenna identifiers. In addition, if the coverage mapsare compressed, the search for the best fit can be isolated to theoverlap between coverage maps associated with the antennas selected tolocate the wireless node.

In another implementation, the RF physical model may be constructedusing an RF prediction model of the coverage area, using mathematicaltechniques like ray-tracing, and the like. In one implementation, the RFprediction model can be computed for each coordinate location in adesired physical space. The estimated signal strength information foreach infrastructure radio transceiver 58 can be used to populate thecoverage maps discussed above. In an alternative embodiment, RFprediction models can be computed relative to each infrastructure radiotransceiver. If path loss symmetry and transmit power symmetry betweenthe wireless nodes and the infrastructure radio transceivers 58 isassumed, the coverage maps for each infrastructure radio transceiverantenna can be populated by using the computed values at each of thecoordinate locations in the coverage map. Of course, site survey datacan also be used to adjust one or more parameters associated with the RFprediction model used to estimate expected signal strength at thevarious locations. As above, the boundaries of the coverage maps can becontoured based on the properties of the antennas connected to theinfrastructure radio transceivers 58.

In addition, the location coordinates in the coverage maps can betwo-dimensional, x- and y-coordinates, defining location in a horizontalplane. The location coordinates can also be three-dimensional, x-, y-and z-coordinates. Other coordinate systems can be used, such asspherical coordinates or cylindrical coordinates. In addition, thevalues of the coordinates can be either global (i.e., longitude andlatitude) or expressed relative to an arbitrarily-defined origin. Inaddition, the granularity of the coordinates in the coverage mapsdepends on the desired granularity of the wireless node locationestimates.

Furthermore, wireless node location module 59, in an alternativeembodiment, can apply other location algorithms, such as a triangulationalgorithm where distances between a given wireless node and three ormore infrastructure radio transceivers 58 are computed based on one ormore path loss exponents and the signal strengths detected by theinfrastructure radio transceivers 58.

Still further, to support dual-band implementations, wireless nodelocation module 59 may maintain location algorithms, such as RF physicalmodels and associated algorithms for more than one band. In anotherimplementation, wireless node location module 59 may maintain differentsets of path loss exponents for each radio frequency band. In oneimplementation, one band may be selected depending on a variety offactors, such as total number of signal strength samples for a givenwireless node, whether the total number is above a threshold or minimumrequired number for estimating location, number of signal strengthsamples over a given threshold, and the like. In other implementations,all bands can be used to compute an estimated location, assuming aminimum number of samples are detected for each band. In otherimplementations, signal strengths values across all bands can be used tocompute the location of the wireless node.

FIG. 2A illustrates a method, according to one implementation of thepresent invention, directed to refreshing signal strength informationfor estimating the location of a wireless node. The wireless nodelocation functionality can be triggered on demand, for example, inresponse to a command issued by a network administrator using a controlinterface to locate a mobile station identified by a MAC address orother suitable identifier, such as an arbitrary name associated with aMAC address in a table or other data structure. Wireless node locationmodule 59 may also be triggered automatically in response to thedetection of a rogue access point. U.S. application Ser. No. 10/407,370,incorporated by reference above, discloses detection of rogue accesspoints in a wireless network system. Wireless node location module 59can also be configured to periodically determine the location of a givenmobile station in order to track its movement over a period of time.

As FIG. 2A illustrates, wireless node location module 59, in oneimplementation, begins by identifying the infrastructure radiotransceivers (IRTs) 58 whose signal measurements will be used inlocating the desired wireless node (102), and collects the signalstrength data from the identified IRTs (104). In one implementation,wireless node location module 59 scans the data structures discussedabove to identify the infrastructure radio transceivers 58 that see ordetect wireless frames transmitted by the desired wireless node.Additional filter criteria can include a threshold signal strengthlevel. If the wireless node has not been seen by any infrastructureradio transceiver 58 (103), wireless node location module 59 reports anerror. Otherwise, in the implementation shown, wireless node locationmodule 59 selects the M infrastructure radio transceivers 58 that reportthe strongest signal strengths in a given band (where M is aconfigurable parameter). In the implementation shown, wireless nodelocation module 59 then determines whether any of the time stampsassociated with the collected signal strength measurements have expired(105). That is, wireless node location module 59, in one implementation,determines whether a sufficient number of infrastructure radiotransceivers 58 have been identified (105). For example, wireless nodelocation module 59 uses the time stamps to filter out infrastructureradio transceivers 58 that have not detected the desired wireless nodewithin a threshold period of time. The exact threshold value is notcritical to the present invention and may also be a configurableparameter. In one implementation, the threshold time period is 30seconds. As FIG. 2A illustrates, wireless node location module 59 thendetermines whether a sufficient number of signal strength values remainfor estimating the location of the wireless node (106). Depending on theimplementation, this minimum number of samples can be arbitrarilyconfigured by a network administrator or system designer, or be requireddue to the inherent requirements of the location algorithm. For example,triangulation requires signal strength samples from at least threeinfrastructure radio transceivers 58. If a sufficient number of samplesremain for analysis, wireless node location module 59 computes theestimated location of the wireless node using any suitable wireless nodelocation model or algorithm, such as the algorithms and models discussedabove.

Otherwise, wireless node location module 59 attempts to refresh thesignal strength information for the wireless node. In implementations,where infrastructure radio transceivers 58 operate as access points,wireless node location module 59, in one implementation, firstidentifies the infrastructure radio transceiver 58 to which the wirelessnode has associated (110). Alternatively, if infrastructure radiotransceivers are solely dedicated to detecting signals for purposes oflocation, wireless node location module 59 identifies the access pointor other WLAN component to which the wireless node is associated. Asdiscussed above, this can be done by querying the WLAN components usingSNMP or other suitable query method. Once the association has beenidentified, wireless node location module 59 transmits a request toterminate the connection with the wireless node to the access pointreporting the association (112). Wireless node location module 59 thenwaits a configurable time, T, to allow the signal strength data torefresh before attempting to compute the estimated location of thewireless node.

In 802.11 wireless networks, the connection with the wireless node canbe terminated in at least two ways. In one implementation, the accesspoint can transmit a deauthentication frame indicating that the accesspoint is terminating the connection. In another implementation, theaccess point can transmit a disassociation frame that terminates theassociation. In either case, selectively terminating the connection inthis manner, causes the mobile station to scan for access points withwhich to associate, as well as transmitting probe responses on allavailable channels in a given band (e.g., 2.4 GHz for 802.11b/gnetworks, and 5 GHz for 802.11a networks), and for dual-band mobilestations transmitting probe requests on all available channels in asecond band. Accordingly, the infrastructure radio transceivers 58within range of the wireless node will ultimately be able to detect theprobe request and its signal strength, regardless of the operatingchannel (and frequency band) to which they may currently be set. Thisscheme also allows the adjacent infrastructure radio transceivers 58 topassively detect the wireless node, and ensures that service to othermobile stations is not interrupted as the infrastructure radiotransceivers 58 need not go off-channel.

FIG. 2B illustrates an alternative process flow according to animplementation of the present invention. In the process flow of FIG. 2B,wireless node location module 59 first scans the association tables todetermine whether the wireless node is associated with an access point(110). If so (111), wireless node location module 59 then terminates thewireless connection as discussed above (112). It then waits for aconfigurable time, T, for the refreshed signal strength information topropagate through the system (114), and gathers signal strength data(102, 104). As FIG. 2B shows, if no association is identified, in oneimplementation, it is assumed that the wireless node is a rogue accesspoint or client. Accordingly, wireless node location module 59 proceedsto collecting the signal strength data, if any, and computes theestimated location of the wireless node (108), assuming a sufficientnumber of signal strength values have been collected.

A variety of embodiments are possible. For example, in an alternativeimplementation, if the wireless node is a rogue client device (which canbe determined by examining the From/To DS bits in the 802.11 frames),wireless node location module 59 could configure the closest radiotransceiver 58 to spoof the rogue access point and transmit adeauthentication and/or disassociation frame. U.S. application Ser. No.10/611,660 discloses the detection of rogue systems and spoofing rogueaccess points to terminate their connections with rogue clients.

B. Integration into Wireless Network Systems

In one implementation, the wireless node location functionalitydiscussed above can be integrated into a wireless networkinfrastructure, such as the hierarchical WLAN system illustrated in FIG.3. For example, the wireless node location functionality describedherein may be integrated into a WLAN environment as disclosed in U.S.application Ser. Nos. 10/155,938 and 10/407,357 incorporated byreference herein. The wireless node location functionality according tothe present invention, however, may be applied to other wireless networkarchitectures. For example, as discussed above, the wireless nodelocation functionality may be integrated into a wireless networkinfrastructure including a plurality of substantially autonomous accesspoints that operate in connection with a central network managementsystem.

Referring to FIG. 3, there is shown a block diagram of a wireless LocalArea Network system according to an embodiment of the invention. Aspecific embodiment of the invention includes the following elements:access elements 11-15 for wireless communication with selected clientremote elements 16, 18, 20, 22, central control elements 24, 25, 26, andmeans for communication between the access elements and the centralcontrol elements, such as direct line access, an Ethernet network, suchas LAN segment 10. As disclosed in U.S. patent application Ser. No.10/407,357, the access elements, such as access elements 11-15 aredirectly connected to LAN segment 10 or a virtual local area network(VLAN) for communication with a corresponding central control element24, 26. See FIG. 3. As disclosed in U.S. patent application Ser. No.10/155,938, however, access elements 11-15 may also be directlyconnected to respective central control elements 24, 26 via directaccess lines.

The access elements 11-15 are coupled via communication means using awireless local area network (WLAN) protocol (e.g., IEEE 802.11a or802.11b, etc.) to the client remote elements 16, 18, 20, 22. Asdescribed in U.S. application Ser. Nos. 10/155,938 and 10/407,357, theaccess elements 12, 14 and the central control element 24 tunnel networktraffic associated with corresponding remote client elements 16, 18; 20,22 via direct access lines or a LAN segment 10. Central control elements24, 26 are also operative to bridge the network traffic between theremote client elements 16, 18; 20, 22 transmitted through the tunnelwith corresponding access elements 11-15. In another implementation,access elements 11-15 may be configured to bridge the network traffic onLAN segments 10, while sending copies of the bridged frames to theaccess elements for data gathering and network management purposes.

As described in the above-identified patent applications, centralcontrol elements 24, 26 operate to perform data link layer managementfunctions, such as authentication and association on behalf of accesselements 11-15. For example, the central control elements 24, 26 provideprocessing to dynamically configure a wireless Local Area Network of asystem according to the invention while the access elements 11-15provide the acknowledgment of communications with the client remoteelements 16, 18, 20, 22. The central control elements 24, 26 may, forexample, process the wireless LAN management messages passed on from theclient remote elements 16, 18; 20, 22 via the access elements 11-15,such as authentication requests and authorization requests, whereas theaccess elements 11-15 provide immediate acknowledgment of thecommunication of those messages without conventional processing thereof.Similarly, the central control elements 24, 26 may for example processphysical layer information. Still further, the central control elements24, 26, as discussed more fully below, may for example processinformation collected at the access elements 11-15 on channelcharacteristics, signal strength, propagation, and interference ornoise.

Central control elements 24, 26, as shown in FIG. 4, may be configuredto gather the signal strength data discussed above to support thewireless node location functionality according to the present invention.The signal strength data gathering functionality described herein isquite similar to the data gathering disclosed in U.S. application Ser.No. 10/183,704, incorporated by reference above. In that application,access elements 11-15 append signal strength data to packets receivedfrom wireless nodes, typically, in encapsulating headers. The centralcontrol elements 24, 26 process the encapsulating packet headers toupdate various data structures, such as the N×N AP signal strengthmatrix and wireless node tables discussed above in Section A. U.S.application Ser. No. 10/183,704 discloses the internal operatingcomponents and general configuration of access elements 11-15 that canbe used in connection with the integrated wireless node locationfunctionality described herein.

FIG. 4 illustrates the logical configuration of central control elements24, 26, according to an implementation of the present invention. Asdiscussed in U.S. application Ser. No. 10/183,704, in oneimplementation, there is both a logical data path 66 and a control path68 between a central control element 24 or 26 and an access element(e.g., access element 11). The control path 68 allows the centralcontrol element 24 or 26 to communicate with the radio access elements11-15 and acquire the signal strength between the radio access elements.By monitoring the data path 66, the central control element 24, 26 canobtain the signal strength of the signals transmitted by other wirelessnodes.

More specifically, the wireless node locator 90 in the central controlelement 24 or 26 collects information from a plurality of accesselements via a control channel 68 and a data channel 66. The centralcontrol element 24 or 26 receives and transmits data packets and controlpackets from/to a plurality of access elements 11-15 as described above.A flag detector 62 distinguishes between data packets and controlpackets, routing them through a logical switch 64 to a high-speed datapath 66 in communication with the wired network 15 or to control path 68within the central control element 24 or 26. The data path 66 ismonitored by a wireless node data collector 70. Associated with eachdata packet is a resource management header which contains RF physicallayer information, such as the power in the channel before each receivedpacket, an identifier for the access element receiving the signal, aswell as an identifier for the antenna selected to receive the signal.This information, together with the 802.11 protocol information in thenative frames, can be used to maintain one or more data structures thatmaintain signal strength data for the wireless nodes detected by theaccess elements 11-15, as discussed in section A, above. The controlpath 68 is coupled to a processor element 76 in which an AP signalstrength matrix 78 is maintained. The AP signal strength matrix 78collects information quantifying the signal strength between accesselements 11-15. All of the signal strength data are collected at theaccess elements 11-15 and communicated over the data path and controlpath to the central control element 24 or 26, in one implementation, aspacketized information in the resource management header in the datapath and resource management control packets in the control path,respectively.

As discussed above, in one implementation, the wireless node locationfunction uses signal strength data between access elements to adjust oneor more parameters of the wireless node location algorithm, or calibratethe signal strength detection across access elements. To support such animplementation, one task is to create and maintain an AP signal strengthmatrix for all the remote access elements in the various wirelessnetworks which detect each other's signals. This is accomplished, in oneimplementation, by having the wireless node locator 90 in the centralcontrol element 24 or 26 and a Resource Manager in the access elements11-15 both passively listen to surrounding access elements and activelyprobe for surrounding access elements. The wireless node locator in thecentral control element 24 or 26 can schedule an access element 11-15 inthe wireless network to transmit a data measurement request on aspecified channel and then record responses from surrounding accesselements. The data measurement probe request and the receiverinformation bandwidth can have a narrower information bandwidth than thenormal information bandwidth in order to allow the dynamic range of thereceiver to be extended beyond its normal operational range. This allowsa radio element to “see” access elements beyond its normal operatingrange. Scheduling these measurements allows multiple measurements to bemade with a single transmission and allows the detection of thetransmitting signal to be recognized as a change in amplitude relativeto the background noise at the scheduled time, allowing for easierdetection of the measurement signal and greater dynamic range. Theresulting data can be transmitted in control packets collected by APsignal strength matrix 78 on the control path 68. Passively, for eachpacket received on the data channel at the access element a measurementof the power in the RF channel is made immediately before the receivedpacket. This interference measurement is sent to the central controlelement via the data channel by appending a Radio Resource Managerheader to the data packet. Alternatively, the access elements may beconfigured to flag packets received from other access elements such thatthey are transmitted on the control path 68.

FIG. 4 also illustrates an RF location model database 80 containing data(such as one or more coverage maps associated with the access elements11-15, the location coordinates of the access elements, path lossexponents, etc.) required by wireless node locator 90 to estimate thelocation of a wireless node. The association tables, discussed above,can either be maintained by the access elements 11-15 individually, orby the central control elements 24, 26 to which the access elements areconnected. When activated, the wireless node locator 90 can operate asdiscussed above to optionally refresh signal information for one or morewireless nodes, as well as compute the estimated location of a desiredwireless node, and return the estimated location to the requestingsystem, such as a network management system or a control interface. Inthe WLAN system depicted in FIG. 3, several implementations arepossible. For example, central control element 24 may be configured as a“master” central control element for purposes of wireless node location.That is, data collected at all central control elements is ultimatelytransmitted (either regularly or on demand) from other central controlelements (e.g., central control element 26) to the master centralcontrol element 24 which controls selective termination of wirelessconnections, and computes the estimated location of, wireless nodes.Alternatively, the collected data can be transmitted to a networkmanagement system that performs the location computations discussedabove. Alternatively, central control elements 24, 26 (when deployed inseparate physical spaces, such as separate floors or buildings) mayoperate substantially autonomously.

The invention has been explained with reference to specific embodiments.For example, although the embodiments described above operate inconnection with 802.11 networks, the present invention can be used inconnection with any wireless network environment. In addition, althoughthe embodiments described above operate in connection with triangulationor RF fingerprinting, any location methodology that relies on signalstrength information associated with wireless nodes can be used in thepresent invention. In addition, although the embodiments described aboveillustrate a system where a dedicated infrastructure performs thelocation determination. The invention described above can be implementedby a client application residing on a wireless node, wherein the clientapplication terminates the connection to refresh signal strengthinformation. Other embodiments will be evident to those of ordinaryskill in the art. It is therefore not intended that the invention belimited except as indicated by the appended claims.

1. In a wireless network environment comprising a plurality of accesselements and at least one wireless node, wherein the wireless node isoperative to transmit wireless frames on a plurality of operatingchannels to discover access points with which to connect, a method forrefreshing signal information in a wireless node location mechanism,comprising receiving a request to estimate the location of a wirelessnode connected to a wireless network; terminating the connection betweenthe wireless node and the wireless network; collecting signal strengthvalues, detected at a plurality of radio receivers, corresponding tosignals transmitted by the wireless node; and computing the estimatedlocation of the wireless node based at least in part on the signalstrength values detected by the plurality of radio receivers.
 2. Themethod of claim 1 wherein the computing step comprises providing thecollected signal strength values to a wireless node location model thatreturns an estimated location for the wireless node.
 3. The method ofclaim 2 wherein the wireless node location model triangulates theestimated location of the wireless node based on the collected signalstrength values and the locations of the plurality of radio receivers.4. The method of claim 1 wherein the wireless network comprises at leastone access point.
 5. The method of claim 4 wherein at least one of theradio receivers is an access point in the wireless network.
 6. Themethod of claim 1 wherein at least one of the radio receivers is anaccess point in the wireless network.
 7. The method of claim 1 furthercomprising wait waiting a period of time, after termination of theconnection between the wireless network and the wireless node, beforecomputing the estimated location of the wireless node.
 8. The method ofclaim 1 wherein the computing step comprises identifying the radioreceivers associated with the signal strengths to be used in locatingthe wireless node; selecting aspects of an RF physical model associatedwith the identified radio receivers; and computing the estimatedlocation of the wireless node using the signal strengths of the signalsdetected by the identified radio receivers, and the selected aspects ofthe physical model.
 9. The method of claim 8 wherein the aspects of theRF physical model are coverage maps corresponding to respective radioreceivers.
 10. The method of claim 9 wherein the coverage maps eachcomprise a plurality of location coordinates associated withcorresponding signal strength values.
 11. The method of claim 10 whereinthe coverage maps are heuristically constructed.
 12. The method of claim10 wherein the coverage maps are based on a mathematical model.
 13. Themethod of claim 1 wherein the wireless node implements the 802.11protocol.
 14. The method of claim 1 wherein the at least one wirelessnode and the radio receivers are capable of operating in more than oneradio frequency band, and wherein the location of the wireless node iscomputed based on the signal strength values detected by the radioreceivers and the radio frequency band associated with the signalstrength values.
 15. The method of claim 14 wherein the computing stepcomprises identifying the radio receivers associated with the signalstrengths to be used in locating the wireless node; selecting aspects ofan RF physical model associated with the identified radio receivers andthe radio frequency band on which the signal strengths were detected bythe radio receivers; and computing the estimated location of thewireless node using the signal strengths of the signals detected by theidentified radio receivers, and the selected aspects of the physicalmodel.
 16. The method of claim 15 wherein the aspects of the RF physicalmodel are coverage maps corresponding to respective radio receivers. 17.The method of claim 16 wherein the coverage maps each comprise aplurality of location coordinates associated with corresponding signalstrength values.
 18. The method of claim 17 wherein the coverage mapsare heuristically constructed.
 19. The method of claim 17 wherein thecoverage maps are based on a mathematical model.
 20. In a wirelessnetwork environment comprising a plurality of access elements and atleast one wireless node, wherein the wireless node is operative totransmit wireless frames on a plurality of operating channels todiscover access points with which to connect, wherein the accesselements are operative to transmit responses to the wireless node, amethod for refreshing signal information in a wireless node locationmechanism, comprising receiving a request to estimate the location of awireless node connected to a wireless network; terminating theconnection between the wireless node and the wireless network;collecting signal strength values of signals transmitted between aplurality of radio receivers and the wireless node; and computing theestimated location of the wireless node based at least in part on thecollected signal strength values.
 21. The method of claim 20 wherein thecollecting step is performed at the wireless node.
 22. The method ofclaim 20 wherein signal strength values are measured at the accesselements.
 23. The method of claim 20 wherein the computing stepcomprises providing the collected signal strength values to a wirelessnode location model that returns an estimated location for the wirelessnode.
 24. The method of claim 23 wherein the wireless node locationmodel triangulates the estimated location of the wireless node based onthe collected signal strength values and the locations of the pluralityof radio receivers.
 25. The method of claim 20 wherein the wirelessnetwork comprises at least one access point.
 26. The method of claim 20further comprising wait waiting a period of time, after termination ofthe connection between the wireless network and the wireless node,before computing the estimated location of the wireless node.
 27. Themethod of claim 20 wherein the computing step comprises identifying theradio receivers associated with the signal strengths to be used inlocating the wireless node; selecting aspects of an RF physical modelassociated with the identified radio receivers; and computing theestimated location of the wireless node using the signal strengths ofthe signals detected by the identified radio receivers, and the selectedaspects of the physical model.
 28. The method of claim 27 wherein theaspects of the RF physical model are coverage maps corresponding torespective radio receivers.
 29. The method of claim 28 wherein thecoverage maps each comprise a plurality of location coordinatesassociated with corresponding signal strength values.
 30. The method ofclaim 29 wherein the coverage maps are heuristically constructed. 31.The method of claim 29 wherein the coverage maps are based on amathematical model.
 32. The method of claim 20 wherein the wireless nodeimplements the 802.11 protocol.
 33. An apparatus facilitating thelocation of a wireless node connected to a wireless network, wherein thewireless node is operative to transmit wireless frames on a plurality ofoperating channels to discover access points with which to connect,comprising a plurality of radio receivers comprising at least oneantenna, the plurality of radio receivers operative to detect thestrength of signals transmitted by wireless nodes and provide thedetected signal strengths to a wireless node location module; and awireless node location module operative selectively terminate theconnection between the wireless node and the wireless network; collectsignal strength values, detected at a plurality of radio receivers,corresponding to signals transmitted by the wireless node; and computethe estimated location of the wireless node based at least in part onthe signal strength values detected by the plurality of radio receivers.34. The apparatus of claim 33 wherein the wireless node location moduleis further operative to wait a period of time, after termination of theconnection between the wireless node and the wireless network, beforecomputing the estimated location of the wireless node.
 35. An apparatusfacilitating the location of a wireless node connected to a wirelessnetwork, wherein the wireless node is operative to transmit wirelessframes on a plurality of operating channels to discover access pointswith which to connect, comprising a communication module operative tointeract with a plurality of radio receivers comprising at least oneantenna, the plurality of radio receivers operative to detect thestrength of signals transmitted by wireless nodes and provide thedetected signal strengths to a wireless node location module; and awireless node location module operative selectively terminate theconnection between the wireless node and the wireless network; collectsignal strength values, detected at a plurality of radio receivers,corresponding to signals transmitted by the wireless node; and computethe estimated location of the wireless node based at least in part onthe signal strength values detected by the plurality of radio receivers.36. The apparatus of claim 35 wherein the communication module comprisesa network interface adapter.
 37. A wireless network system facilitatingthe location of a wireless node, wherein the wireless node is operativeto transmit wireless frames on a plurality of operating channels todiscover access points with which to connect, comprising a plurality ofaccess elements for wireless communication with at least one remoteclient element and for communication with a central control element;wherein a RF coverage map, corresponding to each of the access elements,characterizes the signal strength values for locations in a physicalregion, wherein the access elements are each operative to establish andmaintain, in connection with a central control element, wirelessconnections with remote client elements; detect the strength of receivedsignals; append a signal strength value to frames received from wirelessnodes; and transmit received frames to a central control element; atleast one central control element for supervising the access elements,wherein the central control element is operative to manage wirelessconnections between the access elements and corresponding remote clientelements, and store signal strength data appended to frames transmittedby the plurality of access elements in association with wireless nodeidentifiers; and a wireless node location module operative toselectively terminate the connection between a wireless node and anaccess element; compute the estimated location of the wireless nodebased at least in part on the signal strength values detected by theplurality of access elements.
 38. The system of claim 37 wherein thewireless node location module is further operative to wait a period oftime, after termination of the connection between the wireless node andthe access element, before computing the estimated location of thewireless node.
 39. The system of claim 37 wherein the wireless nodelocation module resides in a network management system.
 40. The systemof claim 37 wherein the wireless node location module resides in thecentral control element.
 41. The system of claim 37 wherein the wirelessnode location module maintains a signal strength matrix including valuesrepresenting the strength of signals detected between the accesselements.
 42. The system of claim 37 wherein the frames are 802.11frames.
 43. The system of claim 38 wherein the wireless node identifiersare MAC addresses.